The objective of the proposed research is to investigate how photoreceptors encode information about changes in light intensity in the presence of background illumination. The long term goal of this research is to arrive at a clearer understanding of the transduction of visual information into neuronal signals. Research will be continued on the hyperpolarizing photoreceptors of the scallop Pectin irradians which are designed to respond to decreases in light intensity under light adapted conditions ("off" receptors) and, therefore, provide a convenient biological model for aspects of vertebrate photoreceptor function. Our results show that the receptor potential of these cells depends on the wavelength as well as on the intensity of light. Experiments are designed to analyze th mechanisms involved in the wavelength dependent response of these cells. Research will be begun on light- and voltage-dependent changes in permeability in isolated rods and cones. Three specific areas will be examined in detail. The intracellular activities of monovalent ions will be determined in rods using ion specific electrodes. The voltage- and time-dependent changes in permeability associated with the decay of the receptor potential will be determined. Finally, changes in light- and voltage-dependent currents in the presence of background illumination will be analyzed.